In some chemical and industrial processes it is desirable to create a stable emulsion of two immiscible liquids (e.g., water and oil). For example, in the field of oil drilling it is useful to force water into an underground space. In order to maximize the amount of oil recovered by this technique, surfactants are used and a stable emulsion is obtained. A surfactant is a molecule that has two portions: one portion is water-soluble (hydrophilic, lipophobic) while the other portion is oil-soluble (hydrophobic, lipophilic). Due to this property of dual solubility, surfactants are able to stabilize emulsions because they bridge the interface between the oil and the water.
Once placed in an oil and water mixture, a surfactant orients itself so that its water-soluble portion is surrounded by water molecules and its oil-soluble portion is surrounded by oil molecules. The mixture is therefore more likely to remain as an emulsion in the presence of a surfactant than it is to separate into its two distinct layers. Thus traditional surfactants are used to stabilize emulsions by preventing them from separating into distinct layers. Stable emulsions are desired in some industrial processes: however, once an emulsion is produced, it is often difficult to break it down and recover the immiscible liquids.
Surfactants are key to many industrial processes in manufacturing and in the energy industry. The careful design of surfactant molecules can greatly facilitate separation processes and thereby decrease the environmental impact of these processes. However, surfactants themselves may cause environmental damage when released to the environment. Even within industrial processes traditional surfactants may cause, rather than solve, separation problems when they stabilize unbreakable emulsions.
Emulsions that are stabilized by traditional surfactants require steps to break the emulsion down and capture the two distinct layers. In some cases, the process that is used to break down the emulsion irreversibly alters the traditional surfactant chemically and makes it ineffective as a surfactant for a second cycle in the process. Where the traditional surfactant is not altered in the emulsion break down process, the waste aqueous solution must be disposed of in a manner that prevents contamination of the environment by the surfactant. An example of the environmental damage that can be caused by surfactants is the reduction of surface tension in natural bodies of water. Even a small amount of surfactant that is released into natural waters will alter the surface tension of the water such that water bugs and mosquitoes are unable to walk upon it. Presence of certain surfactants in bodies of water is toxic to insects and other aquatic life. The result is a lack food for fish and other higher aquatic life, which can significantly alter the food chain.
Such disadvantages may be eliminated by the design and implementation of degradable surfactants. Degradable surfactants have been developed which are designed to degrade after release into the environment, for example, after prolonged exposure to sunlight. This degradation is slow and does not address the environmental contamination that occurs from the time of release to the time of the degradation.
It is desirable to have compounds that act as a surfactant in one form, but can be chemically altered, by a trigger, into another form which does not have surfactant properties. In some cases, it is desirable that the second form act as a demulsifier. An emulsion containing such a surfactant can be broken into its component layers by applying the appropriate trigger to turn off the surfactant. Some known controllable surfactants have cleavable portions. Thus, the trigger causes the surfactant to irreversibly fall apart into two or more fragments, where none of the fragments fulfill the surfactant role of the original molecule. The term “cleavable” is used to indicate such a molecule that is irreversibly changed into two or more fragments. These cleavable surfactants usually cleave slowly over time, and the triggers to cleave them are typically heat or acid. Cleavable surfactants are not suited to reuse or recycling since the cleaving reaction is irreversible.
Other controllable surfactants are “switchable surfactants”. The term “switchable” is used to indicate a molecule that is reversibly changed when a trigger is applied. The switchable surfactant molecule's structure is thus changed to another structure with greatly reduced or even negligible surface activity. In order for the surfactant to be truly switchable, the non-surfactant form of the molecule must be convertible into the surface-active form by the application of another trigger or removal of the first trigger. Examples of known switchable surfactants are those switched “on” (forming the surfactant form) and “off” (forming the non-surfactant form) by triggers such as acid/base cycles, oxidation/reduction cycles, and photochemistry. The applications of these switchable surfactants are limited in some cases because of side reactions caused by the triggering agents. In the case of switchable surfactants that are used to stabilize emulsions, a photochemical switch is inefficient since the emulsions are usually cloudy and/or impermeable to light. Although a cloudy solution can be exposed to light, the photochemical reaction will be slow since the reaction will only occur where the light has effectively penetrated the solution. A further limitation of the known switchable surfactants is that a stoichiometric amount of acid/base or oxidizer/reducer is required, which means a stoichiometric amount of waste is produced. In some examples of surfactant use, such waste is toxic and must be cleaned up before it can be safely released into the environment.
There is a need to have a surfactant that can effectively be reversibly converted between on and off forms using a trigger, preferably a non-toxic trigger. Such a surfactant would stabilize an emulsion when “on” and allow an emulsion to separate into its two phases (or promote such separation) when “off”. Such a surfactant would be suited for recapture, and reuse.